1. Field of the Invention
This invention relates generally to structural poles, and more particularly to composite poles for supporting utility lines.
2. Description of Related Art
Utility lines, such as those carrying electrical power, cable television signals or telephone signals, have traditionally been supported above ground using wooden poles. Trees suitable for use as utility poles, however, have become less available, especially at lengths required for electrical transmission (45 to 120 feet). Because wood is susceptible to insect infestation and rot, it is necessary to constantly replace older wooden poles. The demand for replacement poles, in combination with the demand for new poles, has become increasingly difficult to meet and presents environmental concerns related to deforestation.
Wooden poles pose further environmental hazards. Preservatives such as creosote, used to prevent rot and insect infestation in wooden poles, can seep into groundwater. Environmental groups and the Environmental Protection Agency thus oppose the use of new wooden poles to replace older ones.
Alternative materials have been used in response to the decreasing supply of trees, but these alternatives suffer from their own disadvantages. Metal and concrete poles, for example, are heavy and bulky and therefore difficult to transport or store. The conductivity of metal poles risks arcing between phases, making such poles dangerous to utility personnel servicing lines and necessitating costly methods of insulation. Temperature fluctuations, moisture and salt all cause considerable corrosion to or erosion of these materials.
More recently, fiber/resin composite poles (for example, polyester resin reinforced by glass strands) have been developed to address many shortcomings of metal and concrete poles. Such composite poles are nonconductive, resistant to corrosion and erosion, and are environmentally friendly. Composite poles, however, have not been available at longer lengths for transmission of electrical power and other signals. Current practitioners who are well-versed in state-of-the-art composite tube design and fabrication have found that their designs and processes are restricted to lengths of approximately 45 feet or less. They employ three basic manufacturing approaches.
One approach is to utilize a constant diameter, pultruded shape as a pole. This type of structure is satisfactory for relatively short poles up to approximately 35 feet with diameters of approximately 10 inches. Poles of longer lengths and heavier loading require longer diameters and thicker walls to withstand increased stresses encountered at the base of the pole. For example, a nontapered geometry of a 55-foot pole would require a minimum ground level diameter of 16 inches for a medium-duty pole. End-users resist poles in excess of 10 inches diameter at the top where attachments are made. A pultruded composite pole thus could not be made in accordance with this approach and meet the requirements of the end-user.
A second approach encompasses variations of filament winding techniques with fibers and resins applied about a tapered mandrel. Even though the mandrel is supported at both ends, the combined weight of the mandrel and fiber/resin causes the mandrel to deflect, resulting in bowed or curved portions of the pole when cured and removed from the mandrel. The liquescent nature of the resin precludes supporting the center section during curing. Maximum acceptable bowing restricts the use of this process to producing poles of approximately 45 feet.
A third process attempted involves filament winding additional fiber/resin around pre-existing pultruded tubes. Bowing at cure, described above, also limits the length of pole produced by this process to approximately 45 feet. Thus, fiber/resin poles have not been commercially available at longer lengths suitable for support of electrical transmission lines.
Moreover, a deficiency common to all of the above poles, including traditional wooden poles, is the long lead time required to obtain poles of the required specification and the costs entailed by that delay. Utility carriers have developed exacting structural requirements, tailoring pole specifications for differences in line direction, terrain profile, soil composition, span length, wind velocity, safety needs, degree of insulation and other operationally and environmentally determined factors. With such a variety of specifications, it is very expensive for users to maintain a sufficient inventory of poles to meet unpredictable demand for replacement poles, which demand might result from a storm, for example. High cost of lost revenues from downed lines prohibits waiting for the manufacturer to build poles to specification.